Device for producing stringed instrument or muted horn resonant tones employing a microphone inside or near a speaker enclosure



Apnl 14, 1970 .m. GEORGE 3,506, DEVICE FOR PRODUCING STRINGED INSTRUMENT 0R MUTED HORN RESONANT TONESEMPLOYING A MICROPHOHE INSIDE OR NEAR AVSPEAKER ENCLOSURE 1 2 Sheets-Sheet 1 Filed March 16, 1967 24? 2e M I8 20,

7 7 7 7 I 522 32 SOLOTONE mum I I 2 cm summon mom. mum 2 f TUNING comnouun PORTAMENTO cm AMPLIFIER 05m 40J 42J 44) w ,4: i 350 600 CYCLES 2500 4000' 550 CYCLES 64 750 L5 2000 VIOLIN FUN? "UTED TROMBONE FURMAIT I INVENTOR.

THOMAS .J. HOME ATTORNEYS April 14, 1970 "r. J. GEORGE 3,

DEVICE, FOR PRODUCING STRINGED INSTRUMENT QR MUTED HORN RESONANT TONES EMPLOYING A mcaornoma mswa OR NEAR A SPEAKER ENCLOSURE Filed March 16, 1967 2 Sheets-Sheet 2 INVENTOR. THOIAS J. GEDRGE ATTORNEYS United States Patent 3,506,773 DEVICE FOR PRODUCING STRINGED INSTRU- MENT OR MUTED HORN RESONANT TONES EMPLOYING A MICROPHONE INSIDE OR NEAR A SPEAKER ENCLOSURE Thomas J. George, Burbank, Califl, assignor to Hammond Organ Company, a corporation of Delaware Filed Mar. 16, 1967, Ser. No. 623,615 Int. Cl. G10h 3/00; H04r 5/04 US. Cl. 841.21 14 Claims ABSTRACT OF THE DISCLOSURE An electronic musical instrument for imparting a portamento effect to and modifying the harmonic content of the sound produced thereby; the sound being modified in accordance with a select frequency response or responses comprising sharp-peaks and valleys. The instrument includes first and second audio channels and apparatus for imparting a portamento effect to an input signal applied to the first channel. A standing wave pattern of the signal in the first channel is established and sensed at a fixed location to provide an input signal having a desired harmonic content to the second channel. Accordingly, the first channel provides an open tone sound in accordance with a relatively simple frequency response and the second tone channel provides a resonant tone in accordance with .a complex frequency response having random sharp peaks and valleys, the sounds of the two channels being combined to provide a realistic simulation of certain characteristic sounds such as those of certain musical instruments, including the violin and muted trombone.

BACKGROUND OF THE- INVENTION (1) Field of the invention The present invention relates to electronic musical instruments and more particularly to arrangements for achieving certain tonal effects whereby particular musical instruments or other sounds can be realistically simulated.

(2) Description of the prior art Presently known electronic musical instruments and in particular electronic organs are capable of producing a considerable variety of tonal effects to generate an im.

pressive number of different sounds. Various musical characteristics, such as vibrato, glissando, various harmonic structures, and the shape of the attack and release envelopes may be used to advantage by present day electronic musical instruments. However, no presently known electronic instruments are capable of reproducing certain characteristic sounds such as those of particular musical instruments with the degree of perfection desirable. Certain prior art systems, such as that disclosed in US. Patent No. 3,031,909 to J. P. White, have recognized that such characteristic. sounds, particularly those of certain instruments, require that a very carefully controlled complex wave must be generated within the instrument. In theWhite patent, for example, the audio signalis provided with a select phase .shift so as to produce the throbbing or beating sensation present in many instrument sounds. The phase shift may be accomplished by electronic means or by mechanical resonators which possess the necessary plurality of resonances required to shift the phase of theaudio signal in the desired manner. However, none of the prior art, including the White patent, recognizes the problem of nor provides means for generating an'audio signal having both portamento effect and a harmonic content which is varied in accordance with a frequency response characteristic exhibiting sharp random peaks and valleys. The system disclosed in the White patent does not provide a portamento effect and the audio signal is phase shifted in accordance with a plurality of resonances present in electronic or mechanical resonators.

It has been found in accordance with the present in vention that certain characteristic sounds which heretofore were difficult to realistically simulate can be provided by the advantageous combination of portamento and an audio signal of select harmonic content. Heretofore, little, if anything, was provided by the prior art to impart a portamento effect to an audio signal since it was not recognized that this was one characteristic or effect present in certain sounds. Examples of mechanical and electronic apparatus for providing a portamento effect in conjunction with electronic organs are respectively disclosed in detail in US. Patent No. 3,288,904 of which Thomas I. George is the inventor and in a copending application of Thomas I George, Ser. No. 735,095, filed June 6, 1968, both of which are assigned to the same assignee as the present invention. US. Patent No. 3,288,904 is relied on by the present disclosure for a detailed showing of a part of the apparatus used in the overall combination of the present invention. The audio signal having the necessary portamento effect is modified harmonically in accordance with a desired frequency response. It has been found that by forming a standing wave pattern from the acoustical wave energy produced by a loudspeaker and by sensing the pattern at a fixed location using a microphone, the desired harmonic variation or modification is provided.

\ SUMMARY OF THE INVENTION The present invention provides an electronic musical instrument having first and second audio channels for respectively producing open or string and resonant tone sounds, the open or string tone sound of the first channel being modified in accordance with a desired frequency response characteristic and applied as the input of thesecond channel to produce the resonant tone sound. The input signal to the first audio channel has a frequency determined by the notes played on an associated keyboard, the frequency being changed gradually between different played notes and thereby causing the instrument tone to glide smoothly from one note to another with all intervening frequencies being played in portamento fashion.

In accordance with particular aspects of the invention, the musical instrument may generate characteristic sounds of a stringed instrument or a muted horn by providing interchangeable apparatus for modifying the sound of the first channel in accordance with desired frequency response. characteristics. A standing wave pattern corresponding to that of a stringed instrument may be formed by providing the loudspeaker at the output of the first audio channel with a speaker enclosure and microphone located therein, the microphone converting the acoustical wave energy within the enclosure into an'electrical signal which is passed to the resonant tone audio channel input. Alternatively, ap accurate resonant tone of a muted horn may be generated by disposing a resonator, the resonating properties of -which typify a mute in a position to receive the acoustical wave energy from the speaker at the output of the first audio channel. -A microphone located within the resonator converts the wave energy into an electrical signal which is passed to the resonant tone channel input.

A loundspe'aker with enclosurecoupled to the resonant operation, may be understood from the following description taken in connection with the accompanying drawing, in which:

FIG. 1 is a combination block diagram and schematic representation of a preferred embodiment of an electronic musical instrument in accordance with the invention;

FIG. 2 is a graphical illustration of the frequency responses of the arrangement of FIG. 1 when operating to produce the characteristic sound of a stringed instrument;

FIG. 3 is a harmonic analysis of two different notes produced by the arrangement of FIG. 1 when operating to produce the characteristic sound of a stringed instrument;

FIG. 4 is a graphical illustration of the frequency responses of the arrangement of FIG. 1 when operating to produce the characteristic sound of a muted horn; and

FIG. 5 is a top view of one particular arrangement of an-electronic musical instrument in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As the electronic musical instrument art has progressed, it has become increasingly apparent that merely to duplicate the harmonic structure of the tone from an orchestral instrument is by no means sufficient to accurately simulate the tonal effect of that instrument. It has been found that other factors contribute as much as the harmonic structure of the tone to the realism of the overall tonal effect. Some of these factors include the attack and release of the tone, the shape of the attack envelope, transients occurring within the tone, vibrato, portamento, and any noise which may accompany the tone. Also, there are subtle effects such as harmonic variation from note to note which occur in stringed instruments such as the violin, the cello and the Hawaiian guitar, and in some horns, such as the muted trombone and muted trumpet. Some of these effects are very nearly impossible to measure but can be detected by very careful listening.

The subtle effects of instruments are often individually identifiable only with considerable difficulty, even by the trained ear. Nevertheless, even the untrained ear is immediately aware of the deficiency, if any of the effects are lacking, and is aware at once that the tone is being produced electronically and not by the actual musical instrument itself. Thus, the listener may indicate that the tone heard is like a violin but he does not identify it as a violin. He is not able to say what is lacking, but he knows at once that it is not the actual orchestral instrument that he is hearing. The industry has become accustomed to the situation for so long that the belief prevails that an electronically produced tone can never simulate an actual musical instrument with such accuracy as to be accepted as the instrument itself. However, it has been found that by carefully reproducing the subtle tonal characteristics, as well as the major ones, it is possible to provide the illusion of hearing the actual musical instrument. This is easily proven by having observers listen to tape recordings, and then having them try to identify the instrument they are hearing as being the actual orchestral instrument or an electronic musical instrument. 7 Perhaps the most difficult characteristic to identify in the violin tone, and yet one of the most significant, is the marked changes in the harmonic structure from note to note. It is probable that the acoustical and mechanical resonances present in the body of the violin are chiefly responsible for this fact. It is well known that acoustic and mechanical resonators and vibrating bodies have a providing pleasing string tones, do not produce such tones with the realism necessary to create the illusion of actually hearing a violin. Ordinarily, these string voices are obtained by means of electrical formant circuits, and it is thus quite impossible to obtain the effect of the sharply tuned body resonances in the tones of the violin.

The sharply tuned body resonances such as those present in the violin play a dual function in helping the ear to identify the tone. The first. effect as previously mentioned, is the significant variation in the harmonic tonal structure from note to note. The second, which is closely related to the first, is the acoustic effect which occurs when the violinist plays with portamento, which is so characteristic of violin playing technique. In portamento, as distinguished from glissando, the tone is caused to glide smoothly from one note to another, causing all intervening frequencies to be .played. In glissando, a chromatic sequence of discrete musical notes is played. This difference is quite significant in aiding the ear to identify the tone of the violin, because as the vibration frequency smoothly changes, all body resonances within the frequency range covered by the portamenta are briefly energized, mometarily causing marked changes in the harmonic content of the tone as it glides from note to note. In glissando where discrete frequencies only are played, these frequencies may or may not fall upon one of the resonant frequencies of the violin body, and no briefly passing changes occur in the harmonic structure, though of course the, note to note variations will still be present.

Therefore, to simulate the tones of the violin with any degree of realism, there are at least three tonal characteristics which must be provided with a fair degree of accuracy if the listener is to receive the illusion that he actually is listening to a violin. The first characteristic is that the tone must have a complex waveform, and this is usually obtained by passing a sawtooth wave through a high pass filter. The second required characteristic is that the tones must be playable with portamento, and this presents technical problemsin cases where the tones of the instrument are to be played from a keyboard on which each playing key represents a discrete musical frequency. A satisfactory arrangement for achieving a portamento effect with an electronic organ is disclosed in detail in the previously referred to U.S. Patent No. 3,288,904 of Thomas 1. George. The third characteristic required is that the harmonic structure of the complex Wave vary from note to note. The same requirements apply also to other stringed instruments such as the cello, the viola, the Hawaiian guitar, and also to the muted trombone.

It is Well known that in a room or enclosure of any size and having sound reflective surfaces, a standing wave pattern or so-called acoustic room pattern is established in the presence of a continuously sounding audio tone of fixed frequency. Points of sound reinforcement and other points of sound cancellation occur at various positions throughout the enclosure due to the interferences of the reflected sound waves. Similarly, at any fixed point in the enclosure, numerous reinforcements and cancellations will occur at different frequencies when the frequency of the audio tone is caused to vary causing very sharp peaks and valleys in the frequency response as measured at. the fixed point. It is also well known that the output frequency response of loudspeakers, particularly at the higher frequencies, is accompanied by sharp changes in output sig nal amplitude as'the output frequency varies. To some extent, the same effect, namely variation in output with frequency, is present in certain types of microphones. This is particularly true of those types which do not employ stretched or well damped diaphragms.

On pages 304 and 305 of Applied Acoustics by Olson and Massa, published by Blakiston Son and Co.,' 1939, is 'given' a detailed discussion with frequency response curves of the acoustic room pattern effect. On page 95,0f Applied Acoustics, a response curve of an undamped microphone is shown. 011 pages '58 and 61 of Sound Reproductionf by G. A. Biggs, printed in England by Tapp and Toothill Ltd., 1950, are shown frequency response curves of various loudspeakers measured under laboratory conditions. The many sharp peaks and valleys mentioned above are apparent in all these frequency response curves. It will thus be apparent that when a complex tone, such as a simulated violin tone, is transmitted through such a system, the peaks and valleys in the output frequency response curve will attenuate certain harmonics of the tone and reinforce others, thereby imparting a particular tonal coloration or selective harmonic emphasis, to the audible complex tone. The peaks and valleys are very much sharper than those which can be produced by means of electrical formant circuits, and the effect of the complex tone is thus quite different. An electrical formant circuit usually has a frequency response curve with relatively gradual amplitude changes and thus affects a group of harmonics of the complex tone. However, the peaks and valleys in the response curve of the present invention are so-sharp that individual harmonics are affected. This implies that a small change in'frequency of the complex tone will result in a marked change in the harmonic structure of the tone from the first to the second frequency,

and this is exactly what occurs. The same harmonics are present at both frequencies, but the amplitudes of one or a number of them will have changed substantially, being either greater or less than at the first frequency. This produces a total change which is difficult to define by ear but which is immediately identifiable as the tone of a violin. At each frequency substantially the same harmonics are present, 'but at difierent relative amplitudes.

The same general. principles apply to other stringed instruments such as the cello and Hawaiian guitar, and

also under limited conditions to the muted trombone.

When .used without a mute, the trombone transmits open horn'tones, the coloration of which is determined by the mechanical and acoustical horn structure of the horn and which does not vary substantially from note to note as in the violin. When a mute is added to the horn, open tone is still transmitted but at a lower amplitude. In addition, the sound of the mute itself is added to the open horn tone. The mute is an enclosed metal acoustical resonator, usually having a principal resonant frequency and many minor resonances. In use, it is acoustically coupled to .the open bell of the horn. The point of similarity between the muted horn and violin is that in both the horn mute and the body: of the violin, many sharply tuned points of resonance occur which cause sharp peaks and valleys in the frequency response curves of the instruments. It will also be. apparent that the manner in which the horn mute is used to alter the character of the open horn tone is by applying the mute externally to the horn. Thus, even when using the mute, the horn -still transmits open horn tones, but it is partially altered externally by means of the tuned resonator of the mute.

harmonic structure which are present in the resonant body tone: of the instrument. Thestring itself follows the laws for bowed ,vibrating strings and provides essentially a sawtooth wave at any frequency, while the violin body resonances provide the body tone which exhibits the characteristic peaks and valleys. Thus, the violin and muted horn may be said to generate both open tone and resonant tone simultaneously, and this'efiect is reproduced by the acoustic system disclosed hereinafter.

' Referring now to FIG. 1, a preferred arrangement of an electronic musical instrument in accordance with the present invention is illustrated. A keyboard is arranged by means of a tuning and portamento circuit 12 to control the frequency of a solo tone generator 14 in a mannet which is described in detail in the previously referred to US. Patent No. 3,288,904 of Thomas J. George. Ac-

cordingly, the keyboard 10, the circuit 12, the generator 14 and the various interconnections thereof are illustrated only generally in FIGS. 1 for the sake of clarity. The output signal from the tone generator 14, which has a sawtooth waveform, is applied to a gate 16 which is normally closed but which is coupled to be opened upon the operation of any playing key on the keyboard 10' by switching means well known in the art. The gate 16 is closed when all playing keys are released. When the gate 16 is open, the signal from the tone generator is passed to a formant circuit 18, which by means well known in the art, modifies the waveform of the signal in accordance with whatever instrumental eifect is desired. The modified signal is then amplified by an output amplifier '20 and passed to a loudspeaker 22 for conversion into acoustical wave energy. Thus, when a key on the board 10 is pressed, a solo tone having a frequency determined by the key selected and a tone color detremined by the formant circuit 18 is provided by the loudspeaker 22.

The loudspeaker 22 is mounted in a baffle enclosure 24 which is well known in the art. The enclosure 24, a SIX- sided box, is shown in section and the speaker 22 is mounted in one wall thereof opposite a removable back cover 26. The dimensions of the enclosure 24 are not critical and a volume of approximately 1 cu. ft. has been found satisfactory. An inexpensive crystal microphone cartridge 28 is mounted within the enclosure 24 near the speaker 22 and a resonator chamber 30 1s mounted in front of and close to, but not touching, the diaphragm ofi the speaker 22. The resonator 30 which is shown 1n sectlon may be made of wood or metal, and an open metal can having a diameter of 5 inches and a depth of 3 /2 inches has been found satisfactory. Within the resonator 30 is mounted another crystal microphone 32 which may be similar to the microphone 28.

The microphones 28 and 32 are respectively coupled to the terminals 34 and 36 of a selector switch 38, the wiper arm of which is coupled to a second gate 40. The keyboard switching means (not shown) are coupled to the gate 40 in a manner so as to open the gate upon the operation of any number of playing keys and to close the gate when all keys are released in a manner similar to the operation of the gate 16. The microphones 28 and 32 receive the acoustical wave energy from the loudspeaker 22. The input signal to the gate 40 is therefore selected by the switch 38 and is the signal either from the microphone 28 or 32 depending upon the position of the switch.

With the gate 40 opened, the signal from the selector switch 38 has its waveform modified by a second formant circuit 42 and the modified signal is amplified in a second amplifier 44 and passed to a loudspeaker 46. The speaker 46 is mounted in an enclosure 48 which may be similar in. dimensions to the enclosure 24 and may have an open back. For convenience, the enclosures 24 and 48 may be joined, as indicated by the dotted lines, to form a single structure.

The gate 40 is conventional and may have some adju table means to delay its opening slightly, such as an RC .network in the control circuit. This is not shown since such arrangements are well known in the art. With the wiper arm of the selection switch 38 coupled to the terminal 34, the arrangement of FIG. 1 provides a very realistic violin effect as illustrated in FIG. 2. The formant circuit 18 provides some high frequency emphasis as shown in its frequency response curve A of FIG. 2. Curve A corresponds to the sound from the loudspeaker 22. The signal from the microphone 28 is passed through the gate 40, the formant circuit 42 and the amplifier 44 to the speaker 46. The formant circuit 42 may provide some additional high frequency emphasis if desired, although this is not necessarily required. The principal effect on the tone of the signal in the second audio channel is to introduce very marked frequency distortion. This is in the form of many sharp peaks and valleys in the frequency response curve of the second channel as represented by the curve B of FIG. 2. As previously mentioned, curve B is the summation of peaks and valleys of the response curves of the speaker 22, the microphone 28, and the standing wave pattern or acoustic room pattern existing in the enclosure 24. The acoustic signal levels of the speakers 22 ,soft, the body resonances of the violin are not heard, and

the realism is markedly reduced.

As previously mentioned, when the frequency of the signal provided by the generator 14 changes with a portamento effect, the realism of the violin effect is greatly enhanced. The passing changes in harmonic structure of the tone are responsible for this as illustrated by the harmonic analysis in FIG. 3. FIG. 3 shows the harmonic analysis of the output signal measured at the voice coil of the speaker 46 for two signal frequencies having a difference in fundamental frequency of only 6%. The relative amplitudes of the harmonics of a signal of 392 cycles (middle G) are shown as the solid curve G, and for 415 cycles (G the harmonic amplitudes are shown as the dashed curve Git. Although the two frequencies are only one semitone apart, the difference between the two analyses is very apparent, especially in the region of the 6th,

7th and 8th harmonics. Referring to curve B of FIG. 2,

it will be noted that at the frequencies where these harmonies lie, namely 2.4 kc. to 3.2 kc. there are sharp changes in response in the curve, and this accounts for the marked change in-tone quality apparent both to the ear and by analysis at these two frequencies. No changeln harmonic structure takes place in the signals at the voice coil of the speaker 22.

This selective harmonic emphasis is the kind of tone ,quality change which the ear has learned to associate with the tones of the violin. Curve A of FIG. 2, which could be said to represent the response of the open strlng tone without .the influence of the violin body resonances, exhibits none of the sharp peaks and valleys of curve B. Consequently, the amplitudes of the harmonics for the two signal frequencies will remain virtually unchanged when measured at the voice coil of the speaker 22. Contrasting with this, the amplitudes of nearly all the harmonics have changed, either up or down, when measured at the voice coil of the speaker 46. Thus, when playing portamento from middle G to middle A for example, the harmonic structure of the tone will change markedly as the frequency passes Gil, and this passing change is immediately identified, even by the untrained ear, with violin tone. However, the action which is taking place and which is causing the passing change is quite difficult to identify or define, even by the trained ear, and much observation was required before it was recognized that the passing changes in tone color were the result of momentarily exciting the body resonances of the violin body.

With the wiper arm of the selector switch 38 coupled to the terminal 36, the arrangement of FIG. 1 operates to realistically simulate the effect of a muted trombone as illustrated in FIG. 4. The formant circuit 18 in the first channel, when adjusted to have a frequency response curve with a peak at approximately 700-800 cycles as shown by curve C of FIG. 4, provides a pleasing open trombone tone from the speaker 22. The signal picked up by the microphone 32 within the resonator 30 is amplified in the second channel and transmitted from the speaker 46. The formant circuit 42 may have a frequency response curve similar to curve C of FIG. 4 if desired, but normally this is not necessary. The sharp resonances of the microphone 32 and the resonator 30 have been found sufiicient to provide in the second channel a sharply peaked response curve, as shown by curve D of FIG. 4. The tone from the speaker 46 in the second channel provides a very satisfactory muted trombone sound, and the combined acoustic effect of the sound from both channels is further enhanced, as in the case of the violin, if the opening of the gate 40 is delayed slightly. The dimensions of the'resonator 30 approximate those of a trombone mute, and produce a resonant peak at approximately 640 cycles, as indicated by curve D. Changes in harmonic structure of the muted trombone from note to note, as in the case of the actual muted trombone, are very significant in providing the illusion of actually hearing the orchestral instrument. This is especially true of the passing changes in tone color when the signal frequency changes with portamento, as used in normal trombone playing technique.

As earlier suggested, it may be desirable for convenience to combine the enclosures 24 and 48 into one structure, and when this is done the removable back 26 of the enclosure 24 serves an additional purpose. Although the back 26 is not required, its presence eliminates almost entirely, any feedback between channels. Also, the presence of the back cover 26 seems to cause more peaks and valleys in the response curve B of FIG. 2, and this is desirable. Any audible feedback, however slight, is annoying in the absence of tone signal, that is, when no playing key is pressed. This is then another reason for closing the second gate 40 during the silent intervals between notes.

The two speakers 22 and 46 may be mounted. in the console of the musical instrument if it is desired to dispense with the cost of a separate tone cabinet. FIG. 5 is a top view of a musical instrument console 50 which has a keyboard 52. The speaker 46 is shown mounted in the right end wall 54 of the console and the speaker 22 is mounted in the left end wall 56 with the associated enclosure 24 and microphone 28. Since the speakers 22 and 46 are spaced farther apart in this arrangement, there is less possibility of acoustic feedback, and cover 26 may be dispensed with if desired. It is obvious that other placement of the speakers is possible, such as mounting in the front wall or a rear wall of the console.

The arrangement of FIG. 5 also provides a pseudostereo effect. The pairs of curves in FIGS. 2 and 4 illustrate that the signal from one speaker is always different from that of the other speaker. Since the speakers are separated in this arrangement, the two earswill tend to hear somewhat different sounds, providing a three-dimensional or pseudo-stereo effect. A three-dimensional effect always improves presence which is one aspect of the realism that the present invention seeks to provide.

Although there has been described a specific arrangement of an electronic musical instrument in accordance with the invention for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations or equivalent arrangements falling Within the scope of the annexed claims should be considered to be a part of the invention.

What is claimed is:

1. An electronic musical instrument comprising the combination of means for generating a first electrical signal of complex waveform and having a frequency which represents a desired note on a musical scale, first transducer means for converting the first electrical signal into open acoustical wave energy, means for providing a standing wave pattern of said acoustical wave energy, means for converting the wave energy sensed at a fixed location within the standing wave pattern into a second electrical signal, and second transducer means for converting the second electrical signal into acoustical wave energy.

2. An electronic musical instrument in accordance with claim 1 further including means associated with said generating means for controlling the frequency of said first electrical signal with portamento eifect.

3. An electronic musical instrument in accordance with claim 2 further including formant means for modifying the harmonic content of said first and second electrical signals in accordance with selected frequency responses.

4. An electronic musical instrument in accordance with claim 2 wherein said first and second transducer means respectively comprise first and second loudspeakers and said means for providing the wave pattern and converting it into the second electrical signal comprise a baffle enclosure in which the first loudspeaker is mounted, a first microphone disposed at a fixed point Within the baifle enclosure, a resonator having an open cavity therein and being disposed to receive within the cavity a portion of the acoustical wave energy produced by the first loudspeaker, a second microphone located at a fixed point within the resonator, and switch means coupled to pass one or the other of the signals received in the first and second microphones to the second loudspeaker.

5. An electronic musical instrument in accordance with claim 4 wherein the baffle enclosure is a completely enclosed six-sided structure having a removable back cover, said musical instrument further comprising a second bafile enclosure in which the second loudspeaker is mounted, the second bafile enclosure 'being a five-sided structure and having one wall in common with the first bafiie enclosure.

6. An electronic musical instrument in accordance with claim 2 wherein said means for generating a second electrical signal comprises resonating means responsive to the acoustical wave energy from said first transducer means for providing a standing wave pattern thereof and having random acoustic resonances, and a microphone at a fixed location within the resonating means for converting the acoustical wave energy of the standing wave pattern into said second electrical signal.

7. An electronic musical instrument in accordance with claim 6 wherein said first transducer means includes a loudspeaker and said resonating means includes a baflle enclosure in which is mounted said loudspeaker.

8. An electronic musical instrument in accordance with claim 6 wherein said first transducer means includes a loudspeaker and said resonating means includes a resonator having an open cavity therein, said resonator being disposed to receive within the cavity a portion of the acoustical wave energy produced by said loudspeaker.

9. An electronic musical instrument comprising the combination of a keyboard, solo tone generator means for generating a signal of complex waveform, circuit means interconnecting the keyboard and the generator means for controlling the frequency of the generator means with portamento effect, first formant circuit means for modifying the signal in accordance with a selected frequency response, first gate means for passing the signal to the first formant circuit means only when one or more notes on the keyboard are played, a first loudspeaker for converting the modified signal from the first formant circuit means into acoustical wave energy, a first baffle enclosure in which the first loudspeaker is mounted, a first microphone within the first bafile enclosure for converting acoustic wave energy therein into a corresponding electrical signal, a resonator having a resonating cavity therein and mounted to receive acoustical wave energy from the first loudspeaker within the resonating cavity, a second microphone within the resonating cavity of the resonator for converting the acoustical wave energy therein into a corresponding electrical signal, second gate means for passing an applied signal only when one or more notes on the keyboard are played, switch means for applying the electrical signal from either the first or the second microphone to the second gate means, second formant circuit means for modifying the signal passed by the second gate means in accordance with a selected frequency response, a second loudspeaker for converting the modified signal from the second formant circuit means into acoustical wave energy, and a second bafile enclosure in which the second loudspeaker is mounted.

10. An electronic musical instrument in accordance with claim 9 further including first and second amplifier means for respectively amplifying the modified signals from the first and second formant circuit means, and delay means associated with the second gate means for delaying the passing of a signal by the gate means after the playing of one or more notes on the keyboard.

11. A reproducer for an electronic musical instrument having at least two audio channels for generating the open and resonant tones of a musical instrument sound, said reproducer comprising a loudspeaker coupled to the output of the open tone audio channel, means responsive to the acoustical wave energy produced by the loudspeaker for establishing a standing wave pattern, and a microphone located within the standing wave pattern means and having an output coupled to the input of the resonant tone audio channel.

12. An electronic musical instrument in accordance with claim 11 wherein said microphone is a substantially undamped crystal microphone and wherein said standing wave pattern means comprises a baflie enclosure in which the loudspeaker is mounted.

13. An electronic musical instrument in accordance with claim 11 wherein the microphone is a substantially undamped crystal microphone and the standing wave pattern means comprises a resonator having a resonating cavity, the resonator being positioned to receive a portion of the acoustical wave energy produced by the loudspeaker within the cavity.

14. In an electronic musical instrument wherein a solo tone signal is generated, the combination comprising a cabinet for housing the electronic musical instrument, first and second enclosures within the cabinet, each of the enclosures being a six-sided box, a first loudspeaker mounted in one wall of the first enclosure, a second loudspeaker mounted in one wall of the second enclosure, the first loudspeaker being coupled to receive the solo tone signal, a microphone located at a fixed position within the first enclosure, and circuit means coupling the microphone to the second loudspeaker.

References Cited UNITED STATES PATENTS 2,403,232 7/1946 Parisier l79l.6 2,421,424 6/1947 Kreuzer l79l.6 3,145,265 8/1964 Tamura et a1 l79l.6

HERMAN KARL SAALBACH, Primary Examiner F. BUTLER, Assistant Examiner US. Cl. X.R. 84-1.04; l79l 

